The subject of the present invention is a mixed electronic/O2− anionic conducting composite, its method of preparation and its use as solid electrolyte in a catalytic membrane reactor intended in particular for the production of syngas by the reforming of methane or natural gas.
Catalytic membrane reactors, called CMRs hereafter, produced from ceramics, are used for separating oxygen from air by diffusion of this oxygen in ionic form through the ceramic and the chemical reaction of the latter with natural gas (mainly methane) on catalytic sites (Ni or noble metal particles) deposited on the surface of the membrane. Conversion of syngas into liquid fuel by the GTL (Gas To Liquid) process requires an H2/CO molar ratio of 2. This ratio of 2 can be obtained directly by a process involving a CMR.
However, ceramics are brittle in behavior and have mechanical properties that depend directly on the microstructure (shape and size of the grains, secondary phases, porosity). All other things being equal, the mechanical strength of a ceramic used as a CMR increases when the grain size of which the ceramic is composed decreases. The grain size may increase during operation at temperature and limit the lifetime of the system. Various publications disclose solutions intended to improve this lifetime.
U.S. Pat. Nos. 5,306,411 and 5,478,444 disclose composites consisting of a mixture of an electronic conducting material and an ionic conducting material, thus constituting a solid electrolyte of mixed conductivity.
U.S. Pat. No. 5,911,860 discloses a material essentially consisting of a mixed or ionic conductor and of a constituent with a chemical nature different from the mixed conductor, preferably a metal with a content of 0 to 20 wt %. This publication highlights the need for a second phase in order to limit cracking of the material during sintering and thus to increase its mechanical properties while improving its catalytic efficiency.
U.S. Pat. No. 6,187,157 discloses multiphase systems comprising a mixed ionic/electronic conducting phase or just an ionic conducting phase and a second electronic conducting phase so as to improve the catalytic properties of the material. The secondary phase is generally metallic and occupies 13% of the volume of the material.
U.S. Pat. No. 6,332,964 discloses either a dense membrane or a porous support consisting of a phase comprising a mixed metal oxide of ionic conductivity of the MCeOx, MZrOx type (M: family of lanthanides) or mixed conductivity (LaSrGaMgOx) and of a second phase having an electronic conductivity (metal, metal alloy or mixed oxide of the LaSrMOx type where M=transition element), said second phase being between 1 and 30 vol % of the matrix. U.S. patent application US 2002/0022568 discloses a material of formula Ln1−xSryCax−yMO3−δ (Ln: family of lanthanides and yttrium, or a mixture of the two; M: transition metal or mixture of transition metals) having a high mixed conductivity, a low thermal expansion coefficient and improved mechanical properties. U.S. Pat. No. 6,471,921 discloses a mixed conducting multiphase material whose secondary phases do not participate significantly in the conduction but do increase the mechanical properties of the material. The secondary phases result from a departure from stoichiometric mixing of the precursors used to synthesize the mixed conductor and are therefore by-products of the reaction. The content of secondary phases is between 0.1 and 20 wt %. The main material is a brown-millerite phase of structure AxA′x′A″(2−x−x′)ByB′y′B″(2−y−y′)O5+z and the secondary phases have compositions (A,A′)2(B,B′)O4, A′2(B,B′)O4, (A,A′) (B,B′)2O4, . . . etc. All these secondary phases result from the reaction for synthesizing the material. They are not added before the forming of the material.
The Applicant has sought to develop a composite that has a fine uniform structure with grains having a size close to one micron, thereby guaranteeing high and lasting mechanical properties.